Impact tools fitted with appropriate sockets to tighten or loosen nuts and bolts are well known. However, there is a problem where the same gun is used with a plurality of sizes of bolts. When the gun has sufficient rotary power to tighten large bolts adequately, for example three-quarter inch bolts, then the same gun has the power to twist the head off three-eighths inch bolts under full power. As a result the heads of small bolts are twisted off and great problems are often encountered in removing the broken shaft from the bolt hole, rethreading the bolt hole and reinserting another bolt. It is not practical to have one impact tool for every size of bolt.
In trying to solve the problem where only one gun will be available to the workman one must consider the parameters. In this instance the tool is operating on a source input of about twenty-four to about twenty-eight volts DC, thirty amperes DC, the gun having a one-half inch square drive and a minimum developed torque of about two hundred seventy-five foot pounds.
The first attempt to solve this head shearing problem was to try to control the current in the electrical circuit by use of resistors. Unfortunately, the results were inconsistent and no satisfactory result could be achieved. In particular the impact tool for tightening the bolts was operating on Grade 5 bolts which were being tightened into a metallic bolt hole. The available gun used a hexagonal socket to fit over the head of the bolt and it consistently twisted off the heads of five-sixteenth and three-eighths inch bolts when full power was applied.